This invention relates to an continuous process for the preparation of concentrated aqueous solutions of chlorite salts by the reaction of chlorine dioxide and a reducing agent under reduced pressure to form chlorous acid and subsequently to react the chlorous acid with an aqueous solution of a base.
Chlorites are a class of reagent chemicals which have many commercial uses and many processes for preparation have been used. More recently, the method of choice has been by the reaction of chlorine dioxide, a reducing agent and an alkaline solution. The following U.S. Letters Patent disclose variations of this process:
______________________________________ Inventor(s) Patent No. ______________________________________ Logan 2,046,830 Vincent 2,092,945 Cunningham 2,194,194 Soule 2,332,180 Wagner 2,616,783 Hirschberg et al. 3,101,248 Du Bellay et al. 3,450,493 Callerame 3,828,097 Miller 4,087,515 Lipsztajn et al. 4,915,927 ______________________________________
Logan discloses a process of using chlorine dioxide from gas mixtures to make calcium chlorite. A solution of sodium chloride and calcium chlorate is recycled back to a separator before entry into a reactor.
Vincent discloses preparation of water soluble chlorites by reacting chlorine dioxide with an alkaline solution which contains a reducing agent.
Cunningham discloses preparation of chlorites by the reduction of chlorine dioxide with metallic reducing agents in the presence of an alkaline solution.
Soule discloses preparation of chlorites by the addition of chlorine dioxide to a solution of hydrogen peroxide in an alkaline solution made alkaline with an alkali metal bicarbonate.
Wagner discloses a process for preparing a solid chlorite by reacting chlorine dioxide in an alkaline solution containing a reducing agent, e.g. hydrogen peroxide.
Hirschberg et al. disclose a process for the manufacture of alkali metal chlorides using a mercury amalgam reducing agent in a pH range maintained between pH 7-9. The aqueous solution of sodium chlorite is recycled from a separator to an absorber before entry back to the reactor.
Du Bellay et al. disclose a process of producing alkali metal chlorites in which an alkaline solution of hydrogen peroxide reacts with chlorine dioxide which is bubbled therethrough to yield an alkali metal chlorite. Measurements of redox potential and pH are made, and the reactants are added to a reaction vessel based on the redox and pH measurements.
Callerame discloses a process for preparation of chlorous acid that involves a reaction of a chlorate with a nitrite in a column containing a cation exchange resin.
Miller discloses a process for the production of alkali metal chlorites that involves a reaction of chlorine dioxide with alkali metal amalgams under an atmosphere of nitrogen gas.
Lipsztajn et al., in an electrolytic-electrodialytic process for producing chloric acid, discloses recovery of chloric acid from product line with anolyte being recycled by a line to the water feed line.
All of these above processes are conducted at atmospheric pressure or at a pressure greater than atmospheric pressure.
In those references which use chlorine dioxide, alkali and peroxide, the chlorine dioxide is added to the solution containing peroxide and alkali and the reaction takes place in solution.
One preferred commercial method of manufacturing sodium chlorite starts with the electrochemical formation of sodium chlorate. The sodium chlorate is reduced to form chlorine dioxide. Sodium hydroxide having a low iron content is dissolved in water and hydrogen peroxide is added to the sodium hydroxide solution. The gaseous chlorine dioxide is then added to the solution of sodium hydroxide and hydrogen peroxide. A disadvantage with this method is the size of the equipment required to produce the chlorite. A reaction in solution (liquid/liquid) involves large process liquid volumes to be commercially useful. The kinetics of a liquid/liquid reaction requires a finite time to permit the reaction to be initiated and to proceed to completion. Thus, the reaction must be carried out in large vessels to ensure sufficient residence and contact time for the reactants. Typically, the equipment occupies a volume of approximately 10,000-15,000 cubic feet depending on the plant capacity. Also, the equipment is expensive to procure, install and maintain.
Thus, despite the wide-spread use of these chemicals and the efforts of numerous investigators over a period of years, the commercial production of chlorite is comparatively expensive, requiring large equipment, significant land area, maintenance, large volumes of in-process liquids, and special temperature conditions. Several of the co-inventors of the present invention recognized the need to simplify the process and to provide a more economical product. A process of combining chlorine dioxide and a reducing agent under reduced pressure in a reaction chamber to produce chlorous acid and to combine the chlorous acid with an aqueous solution of base was disclosed in U.S. patent application Ser. No. 08/506,862 filed Jul. 25, 1995. However, further improvements to enable the chlorite to be produced more economically as a saturated solution are needed. The present invention addresses this need.